Project Summary/Abstract Agent Orange and Adverse Birth Outcomes ? A Re-Examination The possibility that the military herbicide Agent Orange caused birth defects in US veterans of the Vietnam War was the subject of two controversial CDC studies in the 1980s and of a number of subsequent studies of US veterans and in the Vietnamese population, some of whose findings have also been questioned. One component of Agent Orange, 2,4-D, continues to be used today and its use is expected to expand. A major source of uncertainty in previous studies is substantial misclassification of exposure due to lack of sound models and accurate exposure data, frequently based on self-report. Under the sponsorship of the National Academy of Sciences we have developed a rigorous and objective exposure assessment system that combines updated and accurate herbicide spray records with extensive military unit location data within an automated GIS system that we also developed. We will apply this methodology to re-assessment of possible relationships between Agent Orange exposure and birth defects in the two CDC studies.